1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, is concerned with an interstage seal arrangement useful between counterrotating airfoil stages of a gas turbine engine.
2. Description of the Prior Art
Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected for driving a rotor which, in turn, drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbofan engines, and turboprop engines.
A recent improvement over the turbofan and turboprop engines is an unducted fan engine such as disclosed in the first U.S. patent application cross-referenced above. In this unducted fan engine, the power turbine includes counterrotating rotors with turbine blades defining counterrotating airfoil stages which drive in corresponding fashion unducted fan blades radially located with respect to the power turbine. The fan blades of the unducted fan engine are variable pitched blades to achieve optimum performance. During operation, fuel efficiency of the engine can be increased by varying the pitch of the blade to correspond to specific operating conditions.
Heretofore, in order to minimize engine performance losses due to leakage of pressure between airfoil stages to the outside of the flow path, labyrinth interstage seals have often been used to deter passage of air. One typical prior art interstage seal is composed of an arrangement of axially spaced circumferentially-extending seal teeth or serrations on the outer ends of the blades which rotate with the engine rotor and a honeycomb component attached to a stationary stator which interfaces with the teeth.
However, this prior art interstage seal arrangement is not suitable for use in the above-described unducted fan engine having the power turbine with counterrotating rotors since successive rows of counterrotating airfoil stages preclude attachment of the honeycomb component to a static or stationary structure. Consequently, a need exists for an interstage seal arrangement which is adapted for use with counterrotating rotors respectively having alternate rows of turbine blades moving in opposite directions.